1. Technical Field
The present invention relates to an optical module for use in the field of optical communication and optical measurement, and more particularly, to an optical module for connecting optical fibers so as to realize functions of optical switch, optical branching, optical coupling, etc., and relates to an optical collimator and a lens housing that are used to constitute such an optical module.
2. Related Art
In optical communication systems and optical measuring systems, a variety of optical connectors and optical modules for connecting optical fibers are utilized. For instance, JP-A-8-114724 discloses an MT (Mechanically Transferable) type optical connector that is adapted to be detachably joined to another optical connector of the same construction by means of guide pins. The two joined optical connectors connect multi-core optical fibers together, which are held by these connectors positioned by the guide pins inserted into guide pin holes formed in the connectors. Distal ends of the multi-core optical fibers extending through the optical connectors are exposed to connecting faces of the connectors, and hence no optical function component part can be placed between opposed end faces of the connectors. Thus, optical transmission alone is permitted between the optical fibers. In other words, the optical connector having the just-mentioned structure can provide neither the optical branching function nor the optical multiplexing function. Besides, the exposed end faces of the multi-core optical fibers can be in contact with each other during operation of connecting the two optical connectors, causing mechanical damages to the fiber end faces.
As for the optical module, it typically comprises two units each of which is adapted to receive end portions of optical fibers for optical transmission between corresponding optical fibers. It is conceivable to construct these units so as to be positioned by means of guide pins as in the MT type optical connector.
FIG. 1 shows such an MT type optical module 1 that comprises two units each comprised of an optical fiber housing (MT ferrule) 2 and a lens housing 3. The optical fiber housing 2 receives distal end portions of two optical fibers 4, and is formed with a pair of guide pin holes 2a. The lens housing 3 is formed with a pair of guide pin holes 3a and a lens hole 3b, with a spherical lens 5 fixed by adhesive at a predetermined position in the lens hole 3b. Each unit has a collimator function for collimating outgoing light beams from the optical fibers by means of the lens 5. The optical fiber housings 2 and the lens housings 3 of the two units are positioned, with the lens housing 3 arranged between the optical fiber units 2, by means of guide pins 6 inserted through guide pin holes 2a and 3a, whereby the optical module 1 is constructed.
For the MT type optical module, the focal distance S from the end face of the optical fiber 4 to the center of the lens 5 and the distance between the centers of the two lenses 5 are important factors affecting on optical characteristics. In order to obtain a proper focal distance S and a proper lens-to-lens distance D, the fiber housing 2 and lens housing 3 must be positioned with submicron accuracy and the lens 5 must be fixed at a predetermined longitudinal position in the lens housing 3 with several-micron accuracy.
In the module 1 using adhesive to fix the lens 5 to the lens housing 3, an error in lens position tends to occur since it is extremely difficult to always constantly apply a minute amount of adhesive to the lens 5 which is extremely small in size. As a result, an applied amount of adhesive varies between lenses 5, causing variations in shrinkage of adhesive and in lens position.
Besides, the optical module 1 does not permit the lens position to change after the lens 5 being fixed. Specifically, the lens-to-lens distance D is kept small if the lens position is deviated in the direction of increasing the focal distance S, whereas the distance D is kept large if the lens position is deviated in the opposite direction. Thus, the dislocated lens position caused by an error in applied amount of adhesive results in inaccuracy of focal distance S and lens-to-lens distance D, producing a large optical coupling loss. Usage of adhesive in fixing the lens 5 entails operations of preparing the adhesive, applying the adhesive to the lens housing 3, and curing the adhesive. This results in an increased number of assembling man-hours to increase assembling costs.
Upon assemblage of the optical module 1, two optical fiber housings 2 are depressed by hands toward two lens housings 3 disposed between the fiber housings, with guide pins 6 inserted through guide pin holes 2a and 3a, whereby these housings 2 and 3 are joined together at their opposed end faces. Since the guide pin holes 2a, 3a have their inner diameters about 0.5 μm larger than the outer diameter of the guide pins 6 to produce clearances between the guide pin holes and the guide pins, it is difficult to stably maintain the joined state (positioned state) of the housings 2, 3. As a result, the focal distance S and the lens-to-lens distance D become unstable between individual optical modules, producing a large variation in their optical coupling characteristics. Besides, the unstable positional relation between optical module components, such as between the lens 5 and the optical fiber 4, can produce a positional deviation that causes a large insertion loss. In particular, when impact is applied to the optical module 1, the lens-to-lens distance D and the focal distance S greatly deviate from design values, causing a large coupling loss and a large insertion loss.
As for an optical module having two optical fiber housings and two lens housings interposed therebetween that are positioned by means of guide pins, it is conceivable to dispose an optical function component, such as a filter, between the lens housings.
To attain a desired optical coupling efficiency in such an optical module, the optical function component must be disposed in accurate position with respect to the lens and at accurate angle with respect to the optical axis. In a case where the optical function component is fixedly positioned by means of the entire end faces of the two lens housings, there is a fear that a desired optical coupling efficiency cannot be attained for the reason that it is practically difficult to machine the entire end face of the lens housing flat enough to attain the desired coupling efficiency from the viewpoints of machining accuracy and machining cost.
Microscopically, the end face of the lens housing, which is not completely flat, has a large number of support points. When the optical function component is fixedly-positioned by means of the entire end faces of the lens housings, the end face of each lens housing abuts against the opposed end face of the optical function component at three supporting points out of a number of support points thereof, and accordingly, the optical function component is positioned from both sides by means of six support points in total. When the optical function component once released from the positioned state is positioned again by means of the same lens housings, it is positioned by six support points that are different from six support points for previous positioning.
Accordingly, with such an arrangement having an optical function component positioned by means of the entire end faces of lens housings that are not completely flat, it is difficult to always position the optical function component at the same position with the same angle. In other words, the position and angle of the positioned optical function component vary between individual optical modules, and hence it is quite difficult to stably manufacture optical modules having a desired optical coupling efficiency.
It is conceivable to construct an MT type optical module as shown in FIG. 2. The optical module 1′ shown in FIG. 2, which is constructed basically the same manner as the optical module 1 shown in FIG. 1, is different therefrom in that a filter 8 is detachably mounted in a spacer 7 disposed between two units, so that the optical module 1′ serves as an optical switch. In FIG. 2, reference numeral 7a denotes a guide pin hole formed in the spacer 7. To permit outgoing light from an optical fiber 4 to propagate along a predetermined optical path, optical fiber housings 2 and lens housings 3 must be positioned with submicron accuracy and a lens 5 must be fixed at a predetermined longitudinal position in the lens housing 3 with several-micron accuracy.
The lens housing for optical module is required to be small in size and low in price, and further requested to satisfy the following requirements:
1) Low insertion loss;
2) Lens face free from being damaged during handling;
3) High optical coupling efficiency between optical fiber and lens;
4) Suppressed reflection loss; and
5) Reduced number of optical module components and manufacturing cost, with improved optical-module assembling efficiency.